Communications interface between host system and state machine

ABSTRACT

A communications interface for interfacing between a host system and a state machine includes an event slot, the event slot comprising a plurality of registers including: a write register for writing by the host system, and a read register for reading by the host system, wherein the event slot is addressed from the host system by a single address location permitting the host system to write data to the write register and/or read data from the read register; and wherein the write register and the read register are individually addressable by the state machine.

CROSS-REFERENCE TO RELATED APPLICATIONS AND CLAIM OF PRIORITY

This application is a continuation under 35 U.S.C. 120 of copendingapplication Ser. No. 15/496,127 filed Apr. 25, 2017, now U.S. Pat. No.10,599,595, which claims foreign priority under 35 U.S.C. 119 fromUnited Kingdom Application No. 1607173.0 filed Apr. 25, 2016.

BACKGROUND

This application relates to circuit architectures. In particular theapplication relates to integrated circuits and communicationsinterfaces.

System designs, such as those for systems-on-chip (SoC), typicallyrequire capabilities of managing multiple tasks and/or managing multiplepower islands or power domains. This can lead to complexity in managingall of these tasks/power islands. Such complexity can arise fromcomplexities in programming operations for multiple power islands andfrom complexities in managing power islands in different contexts, suchas those of SoC with varying levels of complexity themselves: fromInternet-of-Things platforms through to heavy compute platforms.

Typically a central processing unit (CPU) might be used to managetasks/power islands in these situations. However, the CPU may beoverloaded with other processing tasks, and this could lead to systemdelays.

Complexities in systems such as SoC lead to individualised approachesthat can tailor the process management to the particular requirements ofthe relevant system. Such approaches may lead to improvements as regardsthe operation of that particular system. However, these approaches canhave the drawback that they are then no longer applicable to othersystems.

There is a need for an improved integrated circuit. There is also a needfor an improved communications interface.

Such improved integrated circuits and improved communications interfacesare not restricted to managing power islands, but have a more general,or generic, application. For example the improved integrated circuitsand improved communications interfaces can find application in systemsand/or processes in which functions are run in response to signals, forexample signals generated on the occurrence of an event.

BRIEF SUMMARY

There is provided a communications interface for interfacing between ahost system and a state machine, the communications interfacecomprising:

-   -   an event slot, the event slot comprising a plurality of        registers including:        -   a write register for writing by the host system, and        -   a read register for reading by the host system,            wherein the event slot is addressed from the host system by            a single address location permitting the host system to            write data to the write register and/or read data from the            read register; and wherein the write register and the read            register are individually addressable by the state machine.

The event slot may be arranged to communicate with the state machine byplacing at least one event index on an event queue. The event slot maycomprise: a mask register for setting mask values, and a referenceregister for storing reference information.

The reference register may be arranged to comprise a plurality offields, the plurality of fields comprising:

-   -   at least one activate field for controlling the activation of        events,    -   at least one function pointer index field for holding at least        one function pointer index,    -   an interrupt number field for holding an interrupt line number,        and    -   an interrupt enable field for holding an interrupt enable bit.

The at least one activate field may comprise:

-   -   an activate on write field for indicating that a write function        pointer index should be passed to the event queue,    -   an activate on read field for indicating that a read function        pointer index should be passed to the event queue, and    -   an activate on interrupt acknowledge field for indicating that        an interrupt function pointer index should be passed to the        event queue.

The event slot may be at least one of:

-   -   arranged so that the write function pointer index is passed to        the event queue on determining that the host sets any bit in the        write register whose corresponding bit in the mask register is        not set,    -   arranged so that the read function pointer index is passed to        the event queue on determining that the read register is read by        the host, and    -   arranged so that the interrupt function pointer index is passed        to the event queue on determining that the host sets any bit in        the write register whose corresponding bit in the mask register        is set.

The communications interface may comprise a plurality of event slots, atleast two of the plurality of event slots being associated withdifferent address locations.

There is provided an events handler comprising the communicationsinterface as defined above and a state machine configured to execute atleast one function stored in a memory in dependence on data passedthrough the communications interface.

There is provided a power management unit comprising the communicationsinterface as defined above or the events handler as defined above.

There is provided a processing system for processing events, wherein theprocessing system comprises a memory, the memory having located thereinat least a portion of the communications interface as defined above. Thememory may be configured to store a plurality of functions.

There is provided a method of using a communications interface forinterfacing between a host system and a state machine, thecommunications interface comprising an event slot including a writeregister and a read register, the method comprising:

-   -   writing data by the host to the write register at an address        location specified by the host,    -   reading data by the state machine from the event slot,    -   writing data by the state machine to the event slot, and    -   reading data by the host from the read register at the address        location specified by the host.

The communications interface may comprise a reference register and amask register, and the method may comprise at least one of:

-   -   placing a write function pointer index stored in the reference        register on an event queue on determining that the host sets any        bit in the write register whose corresponding bit in the mask        register is not set,    -   placing a read function pointer index stored in the reference        register on the event queue on determining that the read        register is read by the host, and    -   placing an interrupt function pointer index stored in the        reference register on the event queue on determining that the        host sets any bit in the write register whose corresponding bit        in the mask register is set.

The communications interface may comprise a reference register, and themethod may comprise:

-   -   placing, in response to the writing of data by the host to the        write register, a write function pointer index stored in the        reference register on an event queue,    -   reading the write function pointer index from the event queue by        the state machine and executing a write function pointed to by        the write function pointer index.

The communications interface may be embodied in hardware on anintegrated circuit.

There is provided a method of manufacturing, at an integrated circuitmanufacturing system, a communications interface as defined above.

There is provided an integrated circuit definition dataset that, whenprocessed in an integrated circuit manufacturing system, configures thesystem to manufacture a communications interface as defined above.

There is provided a non-transitory computer readable storage mediumhaving stored thereon an integrated circuit definition dataset that,when processed in an integrated circuit manufacturing system, configuresthe system to manufacture a communications interface as defined above.

There is provided an integrated circuit manufacturing system comprising:

-   -   a non-transitory computer readable storage medium having stored        thereon an integrated circuit definition dataset defining the        communications interface as defined above;    -   a layout processing system configured to process the integrated        circuit definition dataset so as to generate a manufacturing        definition of an integrated circuit embodying the communications        interface as defined above; and    -   an integrated circuit generation system configured to        manufacture the communications interface according to the        manufacturing definition.

There is provided computer program code for performing a method asdefined above.

There is provided a non-transitory computer readable storage mediumhaving stored thereon computer readable instructions that, when executedat a computer system, cause the computer system to perform the method asdefined above.

There is provided an integrated circuit configured to perform the methodas defined above.

BRIEF DESCRIPTION OF THE DRAWINGS

Examples will now be described in detail with reference to theaccompanying drawings. In the drawings:

FIG. 1 shows a schematic representation of a portion of a powermanagement unit;

FIG. 2 shows a schematic representation of a communications interface;

FIG. 3 shows another schematic representation of a portion of a powermanagement unit;

FIG. 4 shows a flowchart illustrating a process carried out by anintegrated circuit; and

FIG. 5 shows an example of an integrated circuit manufacturing system.

DETAILED DESCRIPTION

The following description is presented by way of example to enable aperson skilled in the art to make and use the invention. The presentinvention is not limited to the embodiments described herein and variousmodifications to the disclosed embodiments will be apparent to thoseskilled in the art.

The present inventors consider that almost all system-on-chip (SoC)designs (across a wide range of design complexity) will require multiplepower islands or power domains, and correspondingly more advanced powermanagement techniques to meet their target power consumptionconstraints. These constraints are likely to be affected by thermalissues, battery life, legislation and power consumption restrictions(such as in rack mounted servers) amongst others.

The present examples address these issues by providing a genericapproach to an integrated circuit, such as one that could be used inpower management architectures, that can enable easy and consistentintegration into any SoC (or indeed any other suitable system) and whichcan provide reliable and sensible interactions with software operatingsystems.

Whilst a CPU can be used to manage power islands, in some situations theCPU itself might be powered down, so would be unable to manage tasks.There is therefore a need for a ‘lightweight’ controller which can beprovided in an always-on power domain for dealing appropriately withincoming tasks.

It is advantageous to provide the ‘lightweight’ design or system tominimise power consumption during standby conditions and to bearea-optimised so as to take up less semiconductor real estate.Performance in moving between states such as power states is notcritical for many applications, so in these cases power managementoperations can be run at a relatively slower frequency without affectingoverall performance.

The design or system is advantageously programmable and/or flexible.This can mean that it is configurable to suit the needs of a variety ofdifferent platforms, from simple SoCs (such as in Internet-of-Things(IoT) applications) with single switchable power domains and bus-basedinterconnections, through to complex multimedia and compute platformswith many power domains, voltage islands and complex Network-on-Chip(NoC) connectivity. For simple SoCs, it should be possible to hard-codethe required functionality to reduce both hardware and softwarecomplexity.

Further, the design or system is advantageously able to respond both tohardware events, such as power events, and software events, such assoftware event requests. It should be able to move the SoC betweendifferent power states by controlling at least some of power domainvoltage switches, isolation and/or retention cells, the reset state ofpower domains, and clocks, phase-locked loops (PLLs) and analogue IPsamongst others. It should also be able to control power domains externalto the SoC, for example by effecting control of voltage switches and/orvoltage regulators.

The present examples will be discussed in the context of an integratedcircuit (IC) which might be part of a power management system, such as apower management unit (PMU), but it will be understood that theprinciples set out are applicable more broadly than this. For ease ofreference the integrated circuit in the context of the power managementapplication will be referred to below as a PMU IC, though the inventionis not limited to power management applications.

A PMU IC may be located in a power domain that is always switched on (analways-on (or AON) domain). It is therefore always able to run, or be instandby or idle, as required. From the always-on domain the PMU IC cancontrol other power domains by running various functions (so as to turnthe other domains on and off: this can involve sending power-downrequests, receiving acknowledgements, isolating signals and powering theother domains up and/or down).

A PMU IC will have two main tasks: to power up a SoC from a cold reset,and to respond to events (for example power events, in the context of aPMU) which may come from software via the host processor/operatingsystem, such as the central processing unit (CPU), and also fromhardware signalling, by moving the SoC between its various power states,and also by assisting in any other functions required, for example aspart of that power event.

The processing of each power event can follow a unique data flow that isindex-matched to the physical line of a hardware power event request, orthe address used by a software event request. The actions undertaken inresponse to such event requests are a mixture of dedicated power andreset controls, bus accesses (either to peripherals in an always-ondomain, or to the wider system) and via general purpose hardwareinput/output lines.

The PMU IC can achieve this by utilising a single-thread, sequentialinterpreter, such as a state machine, decoding instructions which aretypically stored locally to the PMU IC (but which could also be storedremote from the PMU IC and executed from the remote memory). The statemachine can be task-oriented. The state machine can be small and/or canhave compact code.

The integrated circuit can, through a modification of the mappingbetween at least one signal and at least one function, provide increasedfunctionality whilst minimising or reducing the complexity of theintegrated circuit. This arrangement can reduce power consumption byreducing the processing steps needed, at least for processing onefunction subsequently to the first processing of that function. This canreduce the time taken to subsequently process that function, which canprovide speed enhancements for the integrated circuit.

The structure of the PMU IC will now be briefly described with referenceto FIG. 1. In the illustrated example, a PMU comprises an integratedcircuit which interfaces between a host (CPU) and control interfaces,and also between hardware signalling inputs and the control interfaces.

In one example the integrated circuit comprises a memory 2, a mappinginterface 4 and a state machine 6. In the example illustrated in FIG. 1the integrated circuit also comprises an event queue 8, though it neednot.

The memory 2 comprises a first memory portion 10 and a second memoryportion 12 for storing operational control functions and parameterswhich comprise instructions for the state machine 6. The first memoryportion 10 and the second memory portion 12 are in some examplescontiguous portions of the memory 2. The mapping interface 4 comprises,in this example, both a hardware event interface 14 and a software eventinterface 16. In one example, the hardware event interface 14 isarranged to deal with low-level hardware requests and acknowledgements.In the illustrated example the hardware event interface 14 is connectedto two hardware input lines 15 (though any suitable number can beprovided) for receiving signals from hardware on at least one of theseinput lines.

In one example the software event interface 16 is implemented in a thirdmemory portion of the memory 2 which connects to an input from the hostCPU (illustrated schematically at 24). The third memory portion is insome examples contiguous with the second memory portion 12 in the memory2.

The integrated circuit is arranged in one example so that the statemachine 6 takes data from the mapping interface 4 (i.e. one or both ofthe hardware event interface 14 and the software event interface 16) andprocesses that data in accordance with the instructions stored in thefirst and/or second memory portions, as will be described in more detailbelow.

In the example shown in FIG. 1, the hardware event interface 14 and thesoftware event interface 16 are arranged to pass data to the statemachine 6 via the event queue 8. In other words, the mapping interface 4passes or outputs data to the event queue 8, from where it can be readby the state machine 6. Details of how the data passed to the eventqueue 8 affects the operation of the state machine 6 will be describedin more detail below.

In one example, the first memory portion 10 comprises apower-on-sequence read-only memory (POSROM). This is a non-volatilestructure and will be the fixed entry-point for the state machine 6 oncoming out of reset. In one example, the instructions stored in thePOSROM will turn on the supply switches and deassert the resets for justthe minimum required number of power domains needed to allow the hostCPU to begin booting across all of its boot sequence.

In one example, the second memory portion 12 comprises a read-onlymemory (ROM) or a random access memory (RAM). In some examples thesecond memory portion 12 contains the instructions for executing powermanagement events. These instructions comprise functions in the memory2. The choice of whether to use a ROM or a RAM depends in some exampleson the SoC design constraints.

A ROM offers simple power-management architectures where the number andcomplexity of power events is such that they can be specified duringsystem design. This therefore offers a compact and low powerimplementation. For example, this arrangement can avoid the need forbuilt-in self-test (BIST) collars. Such a ROM could be considered as anextension of the POSROM.

A RAM offers a more flexible power management architecture withpotentially more features. When using a RAM, the state machineinstructions are downloaded to the RAM after host boot-time as a‘firmware’ block. In some examples this can be achieved by a directmemory access (DMA) transfer from non-volatile flash storage. As will beunderstood, this is an example of transferring such a ‘firmware’ block,and other mechanisms could also be used. It is also possible to bootdirectly from volatile memory, for example in a re-boot scenario wherepower is maintained to the volatile memory during the re-boot process.For example, software can be kept in (or loaded into) DDR memory locatedin the always-on domain. This can enable a fast boot, taking advantageof the speed of DDR compared to, for example, flash memory.

The use of a RAM gives the flexibility to update the execution andmapping of power events as part of the host CPU's secondary bootloadersequence, without having to patch any drivers in the operating systemkernel.

A combination of a ROM and a RAM is also possible.

In some examples, the integrated circuit is configured for boot-up orinitialisation from at least one of a local ROM, a local RAM and aremote memory. This can allow firmware initialisation whilst permittingupdating of the firmware as required.

The state machine 6 is, in some examples, the ‘core’ of the PMU IC. Thatis to say, the state machine 6 is arranged to process instructionswithin the PMU IC. The state machine 6 will initially execute theinstructions in the POSROM before moving to a ‘wait’ (or idle) state.The idle state consumes less power than an active state in which thestate machine processes instructions. The state machine 6 need notalways poll the event queue but can be in a quiescent state. Thisarrangement can save power compared to systems which are always in apolling loop or waiting for an interrupt.

Where a RAM is used, the state machine 6 is arranged not to processanything that might be on the event queue 8 until the firmware hascompleted loading in the RAM and the integrated circuit is ready tobegin accepting new events for passing to the event queue 8, for exampleby placing event indices on the event queue. To achieve this, the lastbit of a firmware block, or the last bit of the RAM memory can be usedas, or to set, a ‘firmware ready’ flag. Once set, this flag can indicateto the state machine 6 that the integrated circuit is ready to processevents, and that event indices on the event queue 8 should then beprocessed by the state machine 6. In some examples, this flag is passedto the state machine 6 via line 18.

Thereafter the state machine 6 will parse event requests coming off theevent queue 8, for example as event indices. In some examples theindices are associated with function pointers which point to respectivefunctions in the memory. For example, the indices can comprise therespective function pointers. These indices cause the state machine 6 toexecute the instructions of the respective functions in sequence untilthere is either a jump to another function or a return to the ‘wait’state ready to process the next event index on the event queue 8.

In some examples a function pointer refers (or points) to a function inthe memory (for example by comprising a memory location of thatfunction). That is to say, the event queue 8 is arranged to hold eventindices, which may be the function pointers, to trigger functionexecution, such as for power management events. Thus the particularevent index chosen will cause the state machine 6 to execute a givenfunction. In some examples each index is 7 bits wide, supporting amaximum of 128 function pointers.

The event queue 8 is in some examples a first in first out (FIFO) queue.The depth of the queue, i.e. the number of entries, N, that can be heldon the queue at any one time can be specified as desired, according tothe application. The PMU IC is arranged to determine the number of eventindices, n, on the event queue 8, and to notify the processor, forexample by generating an interrupt when the event queue 8 is full, i.e.when n=N, or an overflow condition has been reached.

In at least some examples, overflow conditions are avoided by monitoringthe number of events and the state of the event queue 8, for example thenumber of events currently pending (i.e. being held on the event queue8). The PMU IC may be configured so as not to pass new events to theevent queue 8 when it determines that the event queue 8 is full.Overflow conditions can be minimised by considering the typical numberof events that are likely to be generated at any one time, and ensuringthat N is sufficiently large to be able to accommodate all such events.

Interactions between the host CPU and the state machine 6 are dealt withthrough the mapping interface 4. In particular the interactions are, inone example, dealt with through a communications interface 20 (shownschematically in FIG. 2). In some examples the software event interfacecomprises the communications interface. In some examples the mappinginterface 4 comprises the communications interface 20. In some examplesthe communications interface 20 is located in the third memory portion.

The communications interface 20 is in at least some examples implementedas a region of flop-based registers and associated logic to manage atleast one of bi-directional data transfer, event triggering andinterrupt support. The number of flops can be minimised withinperformance constraints to increase battery life. In some examples, theregisters are 32-bit registers. The registers are grouped into at leastone event slot 22. In some examples, and as shown schematically in FIG.2, an event slot 22 comprises a group of 4 registers. The at least oneevent slot 22 permits communication between the host CPU and the PMU ICfor the triggering of unique power management events. In some examples,the at least one event slot 22 is the single point of communicationbetween the host CPU and the PMU IC. In some examples the software eventinterface and/or the communications interface is arranged to receive asignal indicative of an event by the writing of information to the eventslot. The event slot 22 will be described in more detail below.

This structure, i.e. that provided by the integrated circuit as outlinedabove, means that the activity required to move between power states isreduced to high level commands from the perspective of the operatingsystem's power management driver. Any SoC-specific control requirements(such as clock programming, off-chip power control, basic hardware setupand process/voltage/temperature (PVT) measurement and/or control) arethen agnostic to the design of the PMU IC, whilst power domain control(such as reset, isolation and power switching) is under the directcontrol of the PMU IC.

Referring again to FIG. 1, the integrated circuit is, in some examples,arranged to receive communications from a host CPU (illustratedschematically at line 24). With reference also to FIG. 3, in oneexample, the communication from the host CPU (at line 26) passes to thememory 2 via a memory arbiter 28. The communication from the host CPUallows the host CPU to download firmware (where applicable) and tocommunicate with the RPU about software event requests (and hardwareevent requests in emulation mode). This also passes through requests toperipherals in the always-on power domain to a first bus master, such asa peripherals bus master in the always-on domain.

The memory arbiter 28 is connected to the state machine 6. The memoryarbiter 28 provides a basic arbitration of internal and external memoryrequests to serve both the memory 2 and control interfaces 32. Thecontrol interfaces 32 comprise system hardware controlling inputs and/oroutputs, which can effect network, power and/or systems control. Thecontrol interfaces 32 are in some examples at least some of versionregisters, general interrupt registers and direct control registers.

The control interfaces 32 are in some examples accessible to both thehost CPU and the state machine 6 and cover hardware version,configuration parameters, debug (host-driven) control and generalinterrupt management.

In some examples, there are a set of general interrupt registers forerror conditions that may be generated by the PMU IC. These includeerrors mentioned above which can be set when the event queue is full.

The mapping interface 4 allows the mapping and re-mapping of addresses,when the firmware is ready, between accesses on page boundaries (wherethe page size is a build-time parameter) and the event slot(s) 22. Thememory 2 also permits monitoring of the communications interface 20 inorder to push event indices onto the event queue 8 and also to manageand generate event-based interrupts. The memory 2 also permits accesscontrols to the rest of the PMU.

Referring again to FIG. 3, in some examples the PMU comprises multiplecontrol interfaces 32, one per switchable power domain. These provide alow-level interface to power switches, isolation control, reset controland local power supervision. For example, each control interface 32 isconnected to a respective power domain. On receiving a command (forexample as part of the processing of a function by the state machine) topower down that power domain, the control interface will in someexamples send a power down request signal to the relevant domain. Thedomain will acknowledge the power down request if it is in a conditionto be powered down. If the power domain is busy and powering down willinterrupt its operation it might either refuse the power down request orsimply not acknowledge it. If the power domain acknowledges the powerdown request, the control interface 32 will isolate the domain to ensurethat spurious signals are not output from the domain. After the domainhas been isolated (and its output(s) entered into known states) thedomain will be powered down.

As shown schematically at 34 in FIG. 3, the control interfaces 32communicate with their respective power domains. The schematiccommunication lines shown at 34 have not been shown on each controlinterface 32 represented for clarity, but it is to be understood thateach control interface 32 is similarly connected to its respectivedomain. Control interface 0 to control interface N−1 are shown in FIG.3, illustrating the instances of control interfaces 32 for N differentpower domains.

In some examples, the integrated circuit is arranged to accessinput/output (IO) control registers (not shown, but which directlyconnect to the state machine 6 in at least some examples). The IOcontrol registers communicate with general purpose input and output(GPIO) lines. The IO control registers are used for manipulating extrahardware interfaces. In some examples the IO control registers are 32bits wide. The IO control registers are in some examples referenced byindex using specific GPIO instructions.

The state machine 6 is, in some examples, a single-stage, single-threadinstruction decoder. In the context of the present description of thePMU IC, the state machine 6 is dedicated to power management functions.The state machine 6 is in some examples based on a 16-bit wide byteaddress/program counter, allowing PMU IC configurations with up to 64 KBof RAM.

The state machine 6 comprises a number of local registers. In someexamples one of these local registers is arranged to hold a 16-bitprogram counter. The state machine 6 is arranged to update the programcounter with a function pointer address whenever an event is availableat the top of the event queue 8. Each instruction in the memory 2pointed to by that address is interpreted and executed in turn, with theprogram counter being incremented by the length of the instruction,unless a jump or return is executed. Instructions are variable-lengthfor code compactness. The instructions are in some examples made up ofat least one of a 6-bit opcode (supporting up to 64 instructions), twocondition flags and optional operand data.

A jump instruction may be a jump to another function in the memory 2.This can allow one set of instructions to refer to another set ofinstructions. Permitting this type of behaviour can allow more complexfunctionality to be built up whilst maintaining code compactness.

A return instruction may be a return to a previous portion of memory 2at which instructions were being executed. In other words, a first setof instructions can be executed, which cause a jump to a second set ofinstructions. At the end of the second set of instructions, a returncommand can cause the state machine 6 to return to the end of the firstset of instructions, and to continue processing from that point.

A stop instruction may cause the state machine 6 to revert to its idlestate, in which it either processes the next event index at the top ofthe event queue 8 or waits in the idle state for another event index tobe placed on the event queue 8.

The mapping interface 4 is configured to perform a mapping from an inputsignal to a function. In other words, the mapping interface can providean output associated with a given function in response to receiving aparticular input, for example a hardware or a software event. In someexamples, the mapping interface comprises a mapping portion 5, which isa portion of the memory 2 that describes the mapping between inputsignals and functions such as the functions stored in the memory 2.

Thus, in some examples, where a signal S is received by the integratedcircuit (S may be indicative of a particular event), the mapping portion5 links that signal to function F, and the mapping interface 4 will mapsignal S to function F.

In some examples it is possible to ‘re-map’ signals (i.e. events) todifferent functions. That is to say, the same event can triggerdifferent functions. For example, a signal S₁ (indicative of event E₁)received by, say, the hardware event interface 14, can be mapped by themapping interface 4 to a function F₁. The mapping interface 4 canperform this mapping by placing an event index associated with functionF₁ on the event queue. Such an event index may comprise a functionpointer FP₁ which points to function F₁. Function pointer FP₁, when readby the state machine, will cause the state machine 6 to look in thememory 2, and to read the instructions at the memory address pointed toby that function pointer (which instructions will in this example befunction F₁).

The mapping interface 4 can re-map signal S₁ such that if it is receivedagain, i.e. subsequent to the first time it is received, that signalwill be mapped to a different function, e.g. function F₂. The mappinginterface 4 can perform this further mapping, i.e. the re-mapping. Themapping interface 4 does this in some examples by placing an event indexassociated with function F₂ on the event queue. Such an event index maycomprise a function pointer FP₂ which points to function F₂. Functionpointer FP₂, when read by the state machine 6, will cause the statemachine to look in the memory 2, and to read the instructions at thememory address pointed to by that function pointer (which instructionswill in this example be function F₂).

In another example, the mapping interface 4 can re-map signal S₁ tofunction F₂ by modifying function pointer FP₁ to point to function F₂rather than pointing to function F₁.

The re-mapping can be performed by the mapping interface 4 by modifyingthe mapping portion 5 which describes the mapping between signals andfunctions.

As an example, the first time an event occurs it is desirable to runsome initialising or setup instructions before running some processinginstructions. Signal S₁ (event E₁) is mapped by the mapping interface 4to function F₁ using function pointer FP₁. FP₁ points to an addresslocation L₁ at which function F₁ (which could be setup instructions) islocated. At the end of function F₁ is a jump instruction to addresslocation L₂ at which function F₂ (which could be other processinginstructions) is located. A stop instruction at the end of function F₂returns the state machine 6 to the idle state.

The first time event E₁ occurs, it is desired to run both functions F₁and F₂. However, in this example, function F₁ need not be run againuntil a reset occurs. The integrated circuit is arranged to re-mapsignal S₁ to achieve this. There are different ways that this can bedone. One example is for function F₁ to include instructions which canbe executed by the state machine 6 to modify function pointer FP₁ topoint to L₂ (i.e. function F₂) instead of to L₁ (i.e. function F₁). Oneexample is for the mapping interface 4 to be arranged to place adifferent event index on the event queue 8 in response to event E₁,which different event index would cause function F₂ to be run.

In other examples, the mapping interface 4 is configured to modify themapping portion 5 in response to receiving signal S₁ so that when signalS₁ is received again it will be re-mapped to a different function(compared to the first time that it is received). In some examples, themapping interface 4, for example the mapping portion 5, comprises asignal counter 17 which can store the number of times that a givensignal has been received. In some examples, the signal counter 17 needonly store a flag or bit which can indicate whether the given signal hasbeen previously received or not.

Where, for example, signal S₁ is received, and the signal counter 17indicates that this signal has not been previously received, the mappinginterface 4 can map S₁ to, say, F₁, and place a corresponding eventindex on the event queue. The mapping interface 4 can then modify thesignal counter 17 to indicate that S₁ has been received, such as bysetting a flag against a bit corresponding to signal S₁. The mappinginterface 4 can also modify the mapping portion 5 to re-map S₁ to F₂(such that the next time S₁ is received it will be mapped to F₂).

Where signal S₁ is subsequently received, the signal counter 17 willindicate that this signal has been previously received, and the mappinginterface 4 can map S₁ to F₂ and place a corresponding event index onthe event queue. This time, the mapping interface 4 need not modify themapping.

Combinations of these approaches are possible. Such combinations areuseful in more complex situations.

The combination used in a situation will depend at least in part onwhether the instructions are held in a ROM or a RAM, which will affectthe place in the process flow at which modifications are able to bemade, such as by the state machine 6 and/or the mapping interface 4.

The re-mapping of event indices and/or function pointers can allow formore complex power state transition tables to be implemented, with ahigher code density and therefore a smaller RAM needed.

In some cases the PMU IC can lock up when processing instructions. Insome examples a PMU IC lock-up can be avoided by a process timeout. Atimeout function loads a value, for example a 32-bit value, into afree-running down-counter and starts it running. In some examples thedown-counter runs off a PMU clock. If the counter reaches zero then themicro-controller will be returned to the idle or ‘wait’ state, as if astop instruction had been reached.

As mentioned above, it is possible to execute functions from memoryexternal to the integrated circuit. In some examples, this is done byway of an instruction in internal memory 2 calling a function inexternal memory. This can be by way of a jump. The instruction ininternal memory 2 can call the function in external memory by specifyingthe memory address at which the external function is located. Theintegrated circuit is in some examples arranged so that the address ininternal memory 2 at which the external function was called is saved asa return point, so that when the external function has completed,operation returns to the memory address of the return point, i.e. backin internal memory 2. Alternatively, a different return point can bespecified to, in effect, jump to another memory address in internal orexternal memory.

This use of external memory allows long functions which might not berequired in standby conditions to be executed from system memory (forexample dynamic voltage and frequency scaling (DVFS) look-ups). Thisminimises the requirements for power-consuming memory in the always-onpower domain (in which the PMU IC is located).

If a function related to a given event is particularly large, it couldrun the risk of blocking the processing of higher priority events on thequeue for a relatively long time. In this case, the function would stillneed to be completed. The function should complete without another eventbeing required to trigger it. However, in some situations it isdesirable to yield to other instructions before carrying on with theexecution of that function. In some examples, this is dealt with bybreaking down that large function into smaller, more manageable,functions. A large function could, for example, be broken into first andsecond smaller functions. Each smaller function can be assigned afunction pointer. The function pointer for the first of the smallerfunctions is, in some examples, the same as the function pointer for thelarge function. At the end of each smaller function (except the finalsmaller function) an instruction can be called to reschedule the nextsmaller function. In other words, to place an event index referring tothat next smaller function on the event queue 8.

Where other event indices are on the event queue 8, these will causeother functions to be executed by the state machine 6 before the next ofthe smaller functions is executed by the state machine 6. Thus, in thisway, the processing of other events is not delayed by the time taken toexecute all of the parts of the large function. When the event indexcorresponding to the next smaller function reaches the top of the eventqueue 8 (which might be immediately, if there are no other event indiceson the event queue 8), it is processed by the state machine 6.

In some examples the function to reschedule the next smaller functionqueries the state of the event queue 8. The querying of the state of theevent queue 8 comprises querying the fill level of the event queue 8.This can comprise querying whether the processor has been notified of anoverflow condition.

An option is to prevent the rescheduling function from rescheduling thenext smaller function of the large event where the event queue 8 isfull, and/or where the processor has been notified of an overflowcondition. Instead the rescheduling function may return the next smallerfunction to the state machine 6 to continue execution.

In this way, the large function can be broken down so as to avoiddelaying other events, but can still be completed if rescheduling is notpossible.

A given function may be considered to be a large function (and so needbreaking down into smaller functions) if it comprises more than athreshold number of instructions and/or processing steps. A givenfunction may be considered to be a large function if it takes (or isestimated to take) more than a threshold time to execute.

If a given function is considered to be a large function, thenoptionally it can be broken down into smaller functions for reschedulingbased on at least one of the following criteria:

-   -   whether the content of the event queue 8 (the number of event        indices on the event queue 8) is above a given threshold;    -   The threshold can comprise at least one of a percentage fill        level and a number of free slots: the threshold is crossed when        the percentage fill level is exceeded, and/or the number of free        slots drops below a given number.    -   If the number of event indices on the event queue 8 is above the        threshold, this can indicate that there are enough other        functions waiting to be executed that it is worth considering        breaking down the large function, so as not to delay the        execution of these other functions.    -   whether the large function is time-critical;    -   If the large function is time-critical then it can be determined        that it should not be broken down and rescheduled, but executed        in one go. Where the large function is not time-critical, it can        be determined that it can be broken down into smaller functions,        at least one of which can then be rescheduled.    -   Whether or not a function is time-critical can be determined        based on, for example, whether at least one of: a given number        of other functions depend on its execution, completion of a task        with at least a given priority level depends on its execution,        and the function itself is associated with at least a given        priority level.

Where it is determined that the large function is to be broken down andat least one of the smaller functions rescheduled, a rescheduleinstruction can be inserted into the large function. This can cause atleast a portion of the large function (i.e. a smaller function) to beplaced at the end of the event queue 8. This can be achieved by placinga function pointer to that smaller function on the event queue 8. Thefunction pointer to that smaller function can be placed on the eventqueue 8 at the stage that the large function is broken down, or duringor at the end of execution of a portion of the large function precedingthe smaller function.

In some examples the function instructions are held in a RAM. The hostCPU would in these examples need to populate the memory 2 with aninitial image comprising a set of instructions for the state machine 6to execute, which are grouped into functions, and also a set of functionpointers which link events to those functions. In some examples thefunctions are hierarchical. This can enable more complex operations tobe built up out of simpler functions.

In some examples, the integrated circuit is arranged to re-map physicaladdresses to a local virtual address space for each businitiator/virtual machine. The integrated circuit can control what isallowed to be accessed in the physical address space. In some examples,this control is done with the granularity of the size of a memorymanagement unit page. The page can be of a range of sizes. The page canbe aligned on any desired boundary, for example 1 KB byte-addressboundaries. This can be fixed at SoC design and defined to the PMU IC bymeans of a given parameter, for example on initialisation.

In some examples, each software event slot 22 is placed on its ownaccess page (and therefore separated by the virtualisation scheme fromall other slots 22). In some examples, the event slots 22 are grouped,for example in powers of 2, into a page where a virtual machinecompartment requires this. This can mean that each virtual machine mayonly access power events made available to it, preventing an ‘insecure’software agent causing undesired power-down events.

In some examples the function instructions are held in a ROM. In theseexamples there is no need to download a firmware image. The PMU IC willbe ready to process power-management events immediately after runningthe power-on reset sequence.

In some examples there is an extension of the ROM. In some examples thisextension is an extra 64 bytes. This can be made up of 16 32-bitregisters. This is to allow patching of the firmware as a backup shouldit be required. In an example an unused function pointer in the ROMpoints to the top of this extension, to enable the extension to beaccessed.

Once the PMU IC has completed its power-on reset sequence and has validfirmware to parse, the host CPU will be able to communicate with the PMUIC via the mapping interface 4, alongside any IPs (or IP cores) involvedwith power state management, to initiate power management tasks(referred to as software events). In addition, such tasks can beconfigured to be executed on hardware flags being set (referred to ashardware events).

Some examples of power management events that could be carried out are:

-   -   power down a power domain or a linked series of power domains at        host request;    -   power up one, many or all power domains on certain IPs flagging        a wake up request;    -   control PMICs or voltage regulators for DVFS/AVS;    -   read registers from a peripheral to decide if a full SoC wake up        is required (such as reading from an IR block);    -   control an external IC's power;    -   use peripherals such as SPI/I2C to control external        ICs/peripherals;    -   configure clock controller;    -   power up or down PLLs and other analogue IPs; and    -   read process/voltage/temperature monitors for DVFS.

The integrated circuit is, in some examples, arranged so that as powermanagement events arrive, whether instigated by software or hardwareevents, they will be placed (i.e. each will cause an event indexrelevant to that event to be placed) on the event queue 8, and processedone at a time.

The state machine 6 may be arranged so that when it has completed itscurrent task it will read the next event index from the event queue 8and carry out its associated task. If there are no event indices in theevent queue 8 then the state machine 6 will enter (or remain in) itsidle state until an event index arrives on the event queue 8.

The RAM and/or ROM contains a number of function pointers which arelinked by indexing to either a hardware event or a write to softwareevent slot (or hardware event slot for debug). The state machine 6 canin some examples only leave its idle state by jumping to an address heldin one of these function pointers. In some examples, each functionpointer holds a 16-bit address of some useful function.

The integrated circuit is in some examples arranged so that on theoccurrence of an event on the hardware event interface 14 (for examplean event on one of the wake-up/sleep interfaces), the state machine 6 isarranged to jump to the respective function pointer.

The integrated circuit is in some examples arranged so that on theoccurrence of a host write to a software event slot 22 (i.e. a softwareevent), the state machine 6 is arranged to jump to the respectivefunction pointer.

As events occur on the hardware and/or software event interfaces, theyare placed on the event queue 8. The event queue is a FIFO structureonto which is placed event indices. Referring to FIG. 4, when the statemachine 6 is in an idle state (and when the firmware ready flag is set(step S1)) it checks to see if there is an event index on the eventqueue (step S2). If so, the state machine 6 reads the next index fromthe event queue (step S3) and reads the corresponding function pointer(step S4) so as to be able to start executing a function pointed to bythat function pointer (at step S5). The state machine 6 continuesprocessing until it reaches the end of the instruction (step S6) when itreturns to its idle state. To this extent, there is in some examples nopriority encoding for power-management events. They are executed on afirst-come, first-served basis, one at a time.

In some examples, the mapping interface 4 is arranged so that ifhardware and software event requests arrive at the same time, thehardware event request is pushed onto the event queue first, with thesoftware event request following it.

In some examples software running on any trusted CPU is able to generatepower management events. This is done by accessing an event slot 22within the mapping interface 4.

In some examples the event slots 22 comprise 4 32-bit registers. Othersizes and/or configurations are possible. The registers can be generic.The use of generic registers can reduce costs. It can also help withscalability as multiple registers of the same kind can be included. Thisarrangement can provide a flexible software interface. It is notnecessary to define beforehand (i.e. before shipping a chip containingthe integrated circuit) all the attributes of the software that will runon this interface. Instead it is possible to decide in the softwareitself how to implement the interface.

The interface (or event slot) operation and/or behaviour is determinedby the firmware image, for example a binary image, that is loaded. It isdesirable for the interface to be compatible with different operatingsystems (OSs). The interface is configurable so that the registers ofthe event slots are compatible with different OSs.

However, preferably an external agent, such as an OS, will not be ableto change the behaviour of the interface or registers. This behaviourshould be a function of the firmware rather than of the OS. The OS needonly see the interface as a communications tool. Thus there need not bevisibility of all of the registers on the OS side: it need only see theregisters with which it communicates. This can simplify the software atthe OS side.

It is also possible for the interface to permit communication betweendifferent portions of a computing system where different securityschemes are employed in each portion. For example, it can control pageaccess over page boundaries. This increases compatibility between suchportions.

The interface can be an interface between an OS and one or more powerdomains.

Each event slot 22 is addressed as a single 32-bit location from theperspective of the host CPU. The number of slots in each memorymanagement page is determined by a system parameter. This can aid there-mapping of system addresses into the PMU IC address map. This canthen allow different virtual machines or secure kernels to have theirown dedicated set or sets of event slots.

Referring to FIG. 2, in some examples each event slot comprises a writeregister 36, a read register 38, a reference register 40 and a maskregister 42. The event slot 22 is arranged so that the host can write tothe write register 36 (illustrated at line 44) and read from the readregister 38 (illustrated at line 46). In one example the host writes toand reads from the respective registers using the single 32-bit locationaddress. In some examples the state machine 6 has read and/or writeaccess to both of the write register 36 and the read register 38. Insome examples the state machine 6 has read and/or write access to eachof the registers in the event slot 22 (illustrated at lines 48). In thissense, the state machine can individually address the registers in theevent slot 22.

In some examples the write register 36 comprises a single 32-bit valuewhich is updated when the host writes to the event slot 22. In someexamples the read register 38 comprises a single 32-bit value which ispopulated by the state machine 6 and read by the host. In some examplesthe host polls the read register 38 to check whether a task is complete.

In one example the mask register 42 comprises a mask value which is usedto determine the setting of interrupts and the calling of interruptmanagement functions. Where a bit is set to ‘1’, if the correspondingbit in the read register 38 is set, an interrupt is triggered. If thecorresponding bit in the write register 36 is set and a correspondingbit in the reference register (an Aw bit—see below) is set, an index toa function pointer (here the write function pointer index) is pushedonto the event queue 8.

In one example the reference register 40 comprises the following fields:

-   -   an activate on write field (Aw)—setting this bit indicates that        the write function pointer index (for example as stored in the        write function pointer index field) will be pushed onto the        event queue 8 when the host sets any bit in the write register        36 whose corresponding bit in the mask register 42 is not set;    -   an activate on read field (Ar)—setting this bit indicates that        the read function pointer index (for example as stored in the        read function pointer index field) will be pushed onto the event        queue 8 when the read register 38 is read by the host;    -   an activate on interrupt acknowledge field (Ai)—setting this bit        indicates that the interrupt pointer index (as stored in the        interrupt pointer index field) will be pushed onto the event        queue 8 when the host sets any bit in the write register 36        whose corresponding bit in the mask register 42 is set;    -   a write function pointer index field—this field comprises a        write function pointer index value which can be pushed onto the        event queue 8 when Aw is set (for example to ‘1’) and the event        slot 22 is written to (i.e. the host writes to the write        register 36). In one example the write function pointer index        comprises an address location (for example in a RAM or a ROM) at        which this function is located. In one example the write        function pointer index comprises an offset from the top of the        function pointers field in the memory 2 (for example in a RAM or        a ROM);    -   a read function pointer index field—this field comprises a read        function pointer index value which can be pushed onto the event        queue 8 when Ar is set (for example to ‘1’) and the event slot        22 is written to (i.e. the state machine 6 writes to the read        register 38). In one example the read function pointer index        comprises an address location (for example in a RAM or a ROM) at        which this function is located. In one example the read function        pointer index comprises an offset from the top of the function        pointers field in the memory 2 (for example in a RAM or a ROM);    -   an interrupt pointer index field—this field comprises an        interrupt function pointer index value which can be pushed onto        the event queue 8 when Ai is set (for example to ‘1’) and the        event slot 22 is written to (i.e. the host writes to the write        register 36). In one example the read function pointer index        comprises an address location (for example in a RAM or a ROM) at        which this function is located. In one example the read function        pointer index comprises an offset from the top of the function        pointers field in the memory 2 (for example in a RAM or a ROM);    -   an IRQ self-clear field—setting a bit in this field will enable        the writing of any bit in the write register 36, where the        relevant mask register 42 bit is also set, to clear the        corresponding bit in the read register 38. This can be used for        immediate IRQ clears without necessarily calling an interrupt        acknowledge function;    -   a copy-back enable field—setting this bit indicates that all        writes to the write register 36 by the host will be immediately        reflected on the read register 38. This emulates the behaviour        of a single read/write configuration register;    -   an IRQ number field—this field specifies the physical IRQ line        to be set when the IRQ conditions for the event slot 22 are met;        and    -   an IRQ enable field—setting this bit enables the generation of        interrupts (which avoids the need for the host to poll the event        slot 22 to determine whether the related task has completed, as        described below).

In at least some examples the Aw, Ar, Ai, IRQ self-clear, copy-backenable and IRQ enable fields are single-bit length fields and theremaining fields are multiple-bit length fields. In some examples asubset of these fields can be used. For example, the IRQ self-clearfield need not be provided as part of the reference register 40.

This configuration allows for a simple event trigger. In some examplesit allows up to 32 bits of parameters to be exchanged between the hostand the PMU IC. In one example, the host writes a parameter to the writeregister 36 of an event slot 22 in which Aw is set and polls the eventslot 22 until the state machine 6 writes a valid response to the readregister 38.

In some examples multiple adjacent event slots 22 can be used. Thisallows for more data to be exchanged. In one configuration, only oneevent slot 22 is allocated as the event trigger by having one or more ofits activate bits (i.e. Aw, Ar and Ai) set. The other slot or slots inthe multiple slots are used to write to the state machine 6 and readfrom the state machine 6. In other words, where an event slot 22comprises 4 registers, all 4 registers are used for the first slot of agroup of multiple slots (which are used to exchange data related to asingle event), whereas only the write registers 36 and read registers 38are used for the remaining slots in the group. These registers can actas ‘overflow’ registers where more than 32 bits (for example) of data istransferred in one go.

In some examples, the host need not poll the read register 38 todetermine when the state machine 6 has completed its task. Such pollingwould consume CPU resources for as long as the task takes to reach theend of the event queue 8 and to be completed by the state machine 6. Anevent can be configured to use a physical interrupt line. In one examplethere are 16 physical interrupt lines which can be used.

The architectural choice of how many of these physical interrupt linesare implemented can be based on the principle of treating eachmanagement event as a separate IP so that each IRQ can effectively beassigned to an event slot 22. This can avoid the need to poll multipleevent slots to discover the source of the interrupt.

For such an interrupt to be enabled, in one example the event slot 22registers are initialised with a bit set in the ‘IRQ enable’ field, anda non-zero value in the mask register 42.

In some examples, interrupt management may be configured along thefollowing lines. The host CPU can write to an event slot 22 and set oneor more bits where the corresponding bits in the mask register 42 arezero. This can represent a data handshake which, if Aw is set, pushesthe stored write function pointer index onto the event queue 8. Whenthis index reaches the top of the event queue 8, the state machine 6executes the relevant function (i.e. the function that is pointed to bythe function pointer) and concludes by writing to the read register 38.The state machine 6 sets any return values, and any interrupt statusbits corresponding to bits set in the mask register 42. If the resultingbitwise AND comparison between the read register 38 and the maskregister 42 is non-zero, the IRQ line corresponding to the value in the‘IRQ number’ field is set.

The host CPU takes the interrupt line specified by the value in the ‘IRQnumber’ field and reads the read register 38 to determine the reasonsfor the interrupt. The host CPU can clear the interrupt source bywriting to any of the bits that are correspondingly set in the maskregister 42.

At this point, one of two mechanisms may operate. If the Ai bit is set(i.e. if it is ‘1’) then the interrupt function pointer index is placedon the event queue 8. This can allow the PMU IC to execute a specificresponse which in some examples includes clearing some, all or none ofthe interrupt bits in the read register 38. If the IRQ self-clear bit isset (i.e. if it is ‘1’) then any interrupt bits that are written to willclear the corresponding bits in the read register 38. Once all bits arecleared, the interrupt line will be deasserted.

In one configuration, the function is expected to clear the writeregister 36, i.e. it is effectively a ‘set’ register. Therefore just theinterrupt clear bits of the slot can be written to (without needing aread-modify-write operation) and not be expected to push the writefunction pointer index onto the event queue 8 again.

Whilst a single event slot 22 corresponds, in some examples, to a givenevent, this need not be the case. In other words, in some examples thereis not a one-to-one mapping between slots 22 and events. Multiple slotswithin a page or across multiple pages may be associated with a singlefunction pointer and/or power event.

In some examples hardware-generated power event requests are hardwareflags from certain IPs which infer that some change of power state isrequired. These can include wake-up events and sleep events. Wake-upevents can typically be interrupts from peripheral IPs in the always-ondomain, such as an infra-red remote receiver, watchdog timer, EthernetMAC (magic-packet wake-up) and panel button, amongst others. Sleepevents can for example include the setting by an RPU of a sleep flagwhich is set when it requires all clocks to be turned off (other thanthe sleep clock).

The hardware event interface 14 is in some examples configured to detectan edge (it can be programmable as to which type: rising, falling, orboth) in an input signal. In some examples the hardware event functionpointers can be re-mapped. A request to this effect may generate an edgethat is detectable by the hardware event interface 14. On determiningsuch a request, the hardware event interface 14 may be configured topush an index matching the request onto the event queue 8, which canre-map one or more function pointers when the event is processed by thestate machine 6. For example this might be by modifying the functionpointer and/or by modifying which function pointer a given signal mapsto. At least one of these modifications can be by the mapping portion 5being modified.

Thus, in some examples, the mapping of a signal to a function can bemodified in response to another signal being received.

In some examples the PMU IC is a single clock-domain IP, operatingnormally from its own dedicated clock source. In some examples the clocksource is shared with the always-on domain's bus clock to reduce theneed for local bus resynchronisation. Under normal conditions, thiswould be expected to be targeted to the frequency of the free-runningoscillator on the PCB, for example a crystal oscillator, to negate theneed for PLLs during stand-by and/or low power operation. In someexamples, for even lower power operation, the PMU's clock source caninclude programmable clock dividers to allow a crystal clock source tobe further reduced in frequency. If higher-performance operation is alsorequired, for example for running DVFS/AVS algorithms during normalrunning, the clock source can optionally come from a fast-clock sourcesuch as a PLL.

In some examples the inputs (general purpose inputs and requests amongstothers) will be synchronised to the PMU clock for metastability. The businterfaces may not have internal synchronisation so this should beprovided in the bus fabric. In some examples outputs from the PMU willbe resynchronised to local clock domains at their end-points.

In some examples the integrated circuit described above is used as alow-cost, generic co-processor for relieving one or more CPU of specificbackground tasks, rather than as a dedicated power controller (forexample running fast, table-based DVFS routines or other lookup/hashroutines).

The integrated circuit/PMU and event slots 22 of FIGS. 1 to 3 are shownas comprising a number of functional blocks. This is schematic only andis not intended to define a strict division between different logicelements of such entities. Each functional block may be provided in anysuitable manner. It is to be understood that intermediate valuesdescribed herein as being formed by the integrated circuit need not bephysically generated by the integrated circuit at any point and maymerely represent logical values which conveniently describe theprocessing performed by the integrated circuit between its input andoutput.

Generally, any of the functions, methods, techniques or componentsdescribed above can be implemented in software, firmware, hardware(e.g., fixed logic circuitry), or any combination thereof. The terms“module,” “functionality,” “component”, “element”, “unit”, “block” and“logic” may be used herein to generally represent software, firmware,hardware, or any combination thereof. In the case of a softwareimplementation, the module, functionality, component, element, unit,block or logic represents program code that performs the specified taskswhen executed on a processor. The algorithms and methods describedherein could be performed by one or more processors executing code thatcauses the processor(s) to perform the algorithms/methods. Examples of acomputer-readable storage medium include a random-access memory (RAM),read-only memory (ROM), an optical disc, flash memory, hard disk memory,and other memory devices that may use magnetic, optical, and othertechniques to store instructions or other data and that can be accessedby a machine.

The terms computer program code and computer readable instructions asused herein refer to any kind of executable code for processors,including code expressed in a machine language, an interpreted languageor a scripting language. Executable code includes binary code, machinecode, bytecode, code defining an integrated circuit (such as a hardwaredescription language or netlist), and code expressed in a programminglanguage code such as C, Java or OpenCL. Executable code may be, forexample, any kind of software, firmware, script, module or librarywhich, when suitably executed, processed, interpreted, compiled,executed at a virtual machine or other software environment, cause aprocessor of the computer system at which the executable code issupported to perform the tasks specified by the code.

A processor, computer, or computer system may be any kind of device,machine or dedicated circuit, or collection or portion thereof, withprocessing capability such that it can execute instructions. A processormay be any kind of general purpose or dedicated processor, such as aCPU, GPU, System-on-chip, state machine, media processor, anapplication-specific integrated circuit (ASIC), a programmable logicarray, a field-programmable gate array (FPGA), or the like. A computeror computer system may comprise one or more processors.

It is also intended to encompass software which defines a configurationof hardware as described herein, such as HDL (hardware descriptionlanguage) software, as is used for designing integrated circuits, or forconfiguring programmable chips, to carry out desired functions. That is,there may be provided a computer readable storage medium having encodedthereon computer readable program code in the form of an integratedcircuit definition dataset that when processed in an integrated circuitmanufacturing system configures the system to manufacture an integratedcircuit or a communications interface configured to perform any of themethods described herein, or to manufacture an integrated circuit or acommunications interface comprising any apparatus described herein. Anintegrated circuit definition dataset may be, for example, an integratedcircuit description.

An integrated circuit definition dataset may be in the form of computercode, for example as a netlist, code for configuring a programmablechip, as a hardware description language defining an integrated circuitat any level, including as register transfer level (RTL) code, ashigh-level circuit representations such as Verilog or VHDL, and aslow-level circuit representations such as OASIS® and GDSII. Higher levelrepresentations which logically define an integrated circuit (such asRTL) may be processed at a computer system configured for generating amanufacturing definition of an integrated circuit in the context of asoftware environment comprising definitions of circuit elements andrules for combining those elements in order to generate themanufacturing definition of an integrated circuit so defined by therepresentation. As is typically the case with software executing at acomputer system so as to define a machine, one or more intermediate usersteps (e.g. providing commands, variables etc.) may be required in orderfor a computer system configured for generating a manufacturingdefinition of an integrated circuit to execute code defining anintegrated circuit so as to generate the manufacturing definition ofthat integrated circuit.

An example of processing an integrated circuit definition dataset at anintegrated circuit manufacturing system so as to configure the system tomanufacture an integrated circuit or a communications interface will nowbe described with respect to FIG. 5.

FIG. 5 shows an example of an integrated circuit (IC) manufacturingsystem 1002 which comprises a layout processing system 1004 and anintegrated circuit generation system 1006. The IC manufacturing system1002 is configured to receive an IC definition dataset (e.g. defining anintegrated circuit or a communications interface as described in any ofthe examples herein), process the IC definition dataset, and generate anIC according to the IC definition dataset (e.g. the IC described, orwhich embodies a communications interface as described, in any of theexamples herein). The processing of the IC definition dataset configuresthe IC manufacturing system 1002 to manufacture the integrated circuitdescribed, or an integrated circuit embodying a communications interfaceas described, in any of the examples herein.

The layout processing system 1004 is configured to receive and processthe IC definition dataset to determine a circuit layout. Methods ofdetermining a circuit layout from an IC definition dataset are known inthe art, and for example may involve synthesising RTL code to determinea gate level representation of a circuit to be generated, e.g. in termsof logical components (e.g. NAND, NOR, AND, OR, MUX and FLIP-FLOPcomponents). A circuit layout can be determined from the gate levelrepresentation of the circuit by determining positional information forthe logical components. This may be done automatically or with userinvolvement in order to optimise the circuit layout. When the layoutprocessing system 1004 has determined the circuit layout it may output acircuit layout definition to the IC generation system 1006. A circuitlayout definition may be, for example, a circuit layout description.

The IC generation system 1006 generates an IC according to the circuitlayout definition, as is known in the art. For example, the ICgeneration system 1006 may implement a semiconductor device fabricationprocess to generate the IC, which may involve a multiple-step sequenceof photo lithographic and chemical processing steps during whichelectronic circuits are gradually created on a wafer made ofsemiconducting material. The circuit layout definition may be in theform of a mask which can be used in a lithographic process forgenerating an IC according to the circuit definition. Alternatively, thecircuit layout definition provided to the IC generation system 1006 maybe in the form of computer-readable code which the IC generation system1006 can use to form a suitable mask for use in generating an IC.

The different processes performed by the IC manufacturing system 1002may be implemented all in one location, e.g. by one party.Alternatively, the IC manufacturing system 1002 may be a distributedsystem such that some of the processes may be performed at differentlocations, and may be performed by different parties. For example, someof the stages of: (i) synthesising RTL code representing the ICdefinition dataset to form a gate level representation of a circuit tobe generated, (ii) generating a circuit layout based on the gate levelrepresentation, (iii) forming a mask in accordance with the circuitlayout, and (iv) fabricating an integrated circuit using the mask, maybe performed in different locations and/or by different parties.

In other examples, processing of the integrated circuit definitiondataset at an integrated circuit manufacturing system may configure thesystem to manufacture the described integrated circuit or communicationsinterface without the IC definition dataset being processed so as todetermine a circuit layout. For instance, an integrated circuitdefinition dataset may define the configuration of a reconfigurableprocessor, such as an FPGA, and the processing of that dataset mayconfigure an IC manufacturing system to generate a reconfigurableprocessor having that defined configuration (e.g. by loadingconfiguration data to the FPGA).

In some embodiments, an integrated circuit manufacturing definitiondataset, when processed in an integrated circuit manufacturing system,may cause an integrated circuit manufacturing system to generate adevice as described herein. For example, the configuration of anintegrated circuit manufacturing system in the manner described abovewith respect to FIG. 5 by an integrated circuit manufacturing definitiondataset may cause a device as described herein to be manufactured.

In some examples, an integrated circuit definition dataset could includesoftware which runs on hardware defined at the dataset or in combinationwith hardware defined at the dataset. The IC generation system mayfurther be configured by an integrated circuit definition dataset to, onmanufacturing an integrated circuit, load firmware onto that integratedcircuit in accordance with program code defined at the integratedcircuit definition dataset or otherwise provide program code with theintegrated circuit for use with the integrated circuit.

The applicant hereby discloses in isolation each individual featuredescribed herein and any combination of two or more such features, tothe extent that such features or combinations are capable of beingcarried out based on the present specification as a whole in the lightof the common general knowledge of a person skilled in the art,irrespective of whether such features or combinations of features solveany problems disclosed herein. In view of the foregoing description itwill be evident to a person skilled in the art that variousmodifications may be made within the scope of the invention.

What is claimed is:
 1. A communications interface for interfacingbetween a host system and a state machine, the communications interfacecomprising: an event slot, the event slot comprising a plurality ofregisters including: a write register for writing by the host system, aread register for reading by the host system, a mask register forsetting mask values, and a reference register for storing referenceinformation, wherein the event slot is addressed from the host system bya single address location permitting the host system to at least one of:write data to the write register, and read data from the read register;and wherein the write register, the read register, the mask register andthe reference register are individually addressable by the statemachine.
 2. The communications interface as claimed in claim 1, whereinthe event slot is arranged to communicate with the state machine byplacing at least one event index on an event queue.
 3. Thecommunications interface as claimed in claim 1, wherein the referenceregister is arranged to comprise a plurality of fields, the plurality offields comprising: at least one activate field for controlling anactivation of events, at least one function pointer index field forholding at least one function pointer index, an interrupt number fieldfor holding an interrupt line number, and an interrupt enable field forholding an interrupt enable bit.
 4. The communications interface asclaimed in claim 3, wherein the event slot is arranged to communicatewith the state machine by placing at least one event index on an eventqueue and wherein the at least one activate field comprises: an activateon write field for indicating that a write function pointer index shouldbe passed to the event queue, an activate on read field for indicatingthat a read function pointer index should be passed to the event queue,and an activate on interrupt acknowledge field for indicating that aninterrupt function pointer index should be passed to the event queue. 5.The communications interface as claimed in claim 4, wherein the eventslot is at least one of: arranged so that the write function pointerindex is passed to the event queue on determining that the host systemsets any bit in the write register whose corresponding bit in the maskregister is not set, arranged so that the read function pointer index ispassed to the event queue on determining that the read register is readby the host system, and arranged so that the interrupt function pointerindex is passed to the event queue on determining that the host systemsets any bit in the write register whose corresponding bit in the maskregister is set.
 6. The communications interface as claimed in claim 1,which comprises a plurality of event slots, at least two of theplurality of event slots being associated with different addresslocations.
 7. The communications interface as claimed in claim 6,wherein a number of event slots is determined by a system parameter. 8.The communications interface as claimed in claim 1, wherein a firmwareimage determines at least one of: an operation of the communicationsinterface, and a behaviour of the communications interface.
 9. Thecommunications interface as claimed in claim 1, wherein thecommunications interface is embodied in an events handler, the eventshandler further comprising a state machine configured to execute atleast one function stored in a memory in dependence on data passedthrough the communications interface.
 10. The communications interfaceas claimed in claim 1, wherein the communications interface is embodiedin a power management unit.
 11. The communications interface as claimedin claim 1, wherein the communications interface is embodied in at leasta part of a memory of a processing system for processing events.
 12. Thecommunications interface as claimed in claim 11, wherein the memory isconfigured to store a plurality of functions.
 13. The communicationsinterface as claimed in claim 1, wherein the communications interface isarranged to control communication between different portions of acomputing system.
 14. The communications interface as claimed in claim13, wherein different security schemes are employed in each portion. 15.The communications interface as claimed in claim 13, wherein eachportion of the computing system comprises a memory management page. 16.The communications interface of claim 1, wherein the communicationsinterface is embodied in hardware on an integrated circuit.
 17. A methodof using a communications interface for interfacing between a hostsystem and a state machine, the communications interface comprising anevent slot including a write register, a read register, a mask registerand a reference which are individually addressable by the state machine,the method comprising: writing data by the host system to the writeregister at an address location specified by the host system, readingdata by the state machine from the event slot, writing data by the statemachine to the event slot, and reading data by the host system from theread register at the address location specified by the host system. 18.The method as claimed in claim 17, the method comprising at least oneof: placing a write function pointer index stored in the referenceregister on an event queue on determining that the host system sets anybit in the write register whose corresponding bit in the mask registeris not set, placing a read function pointer index stored in thereference register on the event queue on determining that the readregister is read by the host system, and placing an interrupt functionpointer index stored in the reference register on the event queue ondetermining that the host system sets any bit in the write registerwhose corresponding bit in the mask register is set.
 19. The method asclaimed in claim 17, wherein the communications interface comprises areference register, the method comprising: placing, in response to thewriting of data by the host system to the write register, a writefunction pointer index stored in the reference register on an eventqueue, reading the write function pointer index from the event queue bythe state machine and executing a write function pointed to by the writefunction pointer index.
 20. A non-transitory computer readable storagemedium having stored thereon an integrated circuit definition datasetthat, when processed in an integrated circuit manufacturing system,configures the system to manufacture a communications interfacecomprising: an event slot, the event slot comprising a plurality ofregisters including: a write register for writing by the host system, aread register for reading by the host system, a mask register forsetting mask values, and a reference register for storing referenceinformation, wherein the event slot is addressed from the host system bya single address location permitting the host system to at least one of:write data to the write register, and read data from the read register;and wherein the write register, the read register, the mask register andthe reference register are individually addressable by the statemachine.